Development of an image display apparatus for displaying an image by scanning a laser beam has been expected in view of the advantage that the image display apparatus is capable of displaying an image with a simplified arrangement in a wide color reproduction range and with a high contrast. In particular, the image display apparatus is suitable for e.g. a mobile projector or an HMD in which miniaturization is required.
For instance, various systems including a system incorporated with a pixel type display device such as a liquid crystal element or an organic EL as an image display portion, and a system of directly rendering an image in the retinas of the user's eyes by two-dimensionally scanning a laser beam have been proposed in an image display apparatus such as an HMD (Head Mount Display), which is configured to be mounted on the head of the user for displaying an image.
In such an image display apparatus, it is necessary to make the entirety of the display apparatus compact and lightweight in order to reduce the load on the user at the time of wearing, and to allow the user to wear the apparatus for a long time. Further, if the image display apparatus is configured substantially with the same design as the generally used eyeglasses, the user is allowed to wear the image display apparatus on a steady basis like the usual eyeglasses.
However, in the system incorporated with the pixel type display device, as a higher quality and a wider viewing angle are required, a display portion, a prism for guiding light emitted from the display portion to the eyes, and an eyepiece optical system incorporated with a half mirror increase in size. This makes it difficult to make the image display apparatus compact and lightweight.
Further, the large-sized eyepiece optical system as described above is configured to cover the space in front of the eyes, and has a shape similar to the shape of a goggle or a helmet, rather than the shape of eyeglasses. Thus, the user finds it difficult to feel as if he or she wears eyeglasses, and it is difficult to implement a general eyeglass type image display apparatus.
On the other hand, a retinal scanning type image display apparatus employing a laser scanning system is advantageous in miniaturizing the image display apparatus because of the use of a compact MEMS (Micro-Electro-Mechanical-System) mirror device.
It is suitable to use a resonant mirror capable of generating a large displacement with a small driving force in an image display apparatus which is required to be miniaturized such as the aforementioned eyeglass type HMD. Further, it is optimal to use a two-axis resonant MEMS mirror capable of scanning along two axes with a one-chip device.
Usually, in the case where a laser beam is scanned by a scanning mirror, it is desirable to scan at a high speed in a horizontal direction, and at a speed suitable for the frame rate of displaying a moving image in a vertical direction, e.g., at 60 Hz. However, designing a scanning mirror to resonate at such a low frequency as described above may lower the rigidity of a spring structure for supporting the mirror, and may weaken the mirror against external disturbance such as vibrations.
In view of the above, there is proposed a method for resonantly driving a scanning mirror along two axes at a higher frequency, in other words, a method for displaying an image by Lissajous scanning.
For instance, patent literature 1 discloses a relationship between a frequency and a phase in displaying an image by Lissajous scanning, using a two-axis resonant scan system.
Patent literature 1 further discloses the following. Usually, since a sinusoidal wave scan pattern does not intersect the position of a source pixel derived from a source image, the position of a scanning pixel does not coincide with the position of the source pixel. Accordingly, the quality of a scan image is improved by interpolating the intensity of a scanning pixel by an image generator based on the intensity of a source pixel.
The conventional arrangement has the following problems to be solved.
It is necessary to access pixel data of plural source pixels and to read out the pixel data from a buffer memory storing the pixel data of source pixels in order to interpolate pixel data of a source image with respect to the position of a certain scanning pixel for determining the pixel value of the scanning pixel.
Usually, a DRAM (a Dynamic RAM) is used as a source image buffer memory for an image display apparatus.
A DRAM has a property that stored information is lost as time elapses. Accordingly, it is necessary to rewrite information of the same content before the information is lost (a refresh operation).
Further, in the case where a data read-in request is issued from an external device with respect to a certain address within a DRAM memory chip, data stored in a memory cell of the DRAM is lost simply by receiving an input signal indicating the request. In view of the above, a sense amplifier is provided as a measure against an external stimulus. There is performed a processing of returning the data evacuated in the sense amplifier back to a corresponding row in the memory array (a pre-charge operation).
In contrast, an SRAM (a Static RAM) has a property that once information is written, the written information is retained as long as electric power is continued to be supplied. In other words, the information is retained, even if a refresh operation or a pre-charge operation is not specifically performed. Thus, any address data is accessible at any time.
However, since an SRAM is comprised of a flip-flop circuit including at least four transistors per memory cell, the number of wirings is large, and it is difficult to increase the capacity. On the other hand, since a DRAM is comprised of one transistor and one capacitor, and has a simple structure, it is easy to manufacture the DRAM at a low cost and with a large capacity. In view of the above, a DRAM is used for a device requiring a large capacity, such as a main memory of a personal computer or an image buffer memory.
A DRAM has a burst transfer mode, and high-speed processing is achieved by designating an address (a leading address) and by sequentially transferring data to addresses succeeding the leading address. In the case where data is randomly accessed to a certain address, a time for the aforementioned refresh operation or pre-charge operation is required. As a result, a time required for a data readout operation increases, as compared with a case where an SRAM is used.
In an image display apparatus configured such that Lissajous scanning is performed using a two-axis resonant MEMS mirror, as employed in the present invention, the address direction of data stored in an image buffer memory is different from an actually displayed scanning direction. Accordingly, a data readout operation into the image buffer memory becomes substantially a random access operation, and the speed of displaying a scanning pixel is restricted by the pixel data readout speed from the memory.
It is necessary to perform memory access a certain number of times for generation of an address at one pixel in order to read out plural pixel data for pixel data interpolation, and a certain access time is required. As a result, it is difficult to raise the display speed of a scanning pixel. In other words, if an interpolation processing is implemented by a memory system having the same transfer rate, the display speed of a scanning pixel resultantly lowers, which makes it difficult or impossible to raise the display resolution over a predetermined value.
The conventional example as described in patent literature 1 fails to disclose an improvement on the memory access speed.